$\begin{array}{l}\quad A=\dfrac4{7.31}+\dfrac6{7.41}+\dfrac9{10.41}+\dfrac7{10.57}\\\to\dfrac A5=\dfrac4{7.31.5}+\dfrac6{7.41.5}+\dfrac9{10.41.5}+\dfrac7{10.57.5}\\\to\dfrac A5=\dfrac4{31.(7.5)}+\dfrac6{(7.5).41}+\dfrac9{41.(10.5)}+\dfrac7{(5.10).57}\\\to\dfrac A5=\dfrac4{31.35}+\dfrac6{35.41}+\dfrac9{41.50}+\dfrac7{50.57}\\\to\dfrac A5=\dfrac1{31}-\dfrac1{35}+\dfrac1{35}-\dfrac1{41}+\dfrac1{41}-\dfrac1{50}+\dfrac1{50}-\dfrac1{57}\\\to\dfrac A5=\dfrac1{31}-\dfrac1{57}\quad(1)\\\quad B=\dfrac7{19.31}+\dfrac5{19.43}+\dfrac3{23.43}+\dfrac{11}{23.57}\\\to\dfrac B2=\dfrac7{19.31.2}+\dfrac5{19.43.2}+\dfrac3{23.43.2}+\dfrac{11}{23.57.2}\\\to\dfrac B2=\dfrac7{31.(19.2)}+\dfrac5{(19.2).43}+\dfrac3{43.(23.2)}+\dfrac{11}{(23.2).57}\\\to\dfrac B2=\dfrac7{31.38}+\dfrac5{38.43}+\dfrac3{43.46}+\dfrac{11}{46.57}\\\to\dfrac B2=\dfrac1{31}-\dfrac1{38}+\dfrac1{38}-\dfrac1{43}+\dfrac1{43}-\dfrac1{46}+\dfrac1{46}-\dfrac1{57}\\\to\dfrac B2=\dfrac1{31}-\dfrac1{57}\quad(2)\\\text{- Từ (1) và (2) $\to\dfrac A5=\dfrac B2\\\to\dfrac AB=\dfrac52$} \end{array}$
